The present invention relates to an animal feed containing carboxylic acids either as the sole active ingredient or in combination with a bioactive 1.alpha.-hydroxy vitamin D compound and/or the enzyme phytase. The use of these carboxylic acids, preferably citric acid, in animal feeds removes calcium from plant based phytate complexes thereby allowing the phytate to solubilize making the phosphorus contained therein bioavailable for animals. These carboxylic acids also increase utilization of the phosphorus available from inorganic sources in the diet. This results in a feed composition having a severe reduction of, and possibly the complete elimination of, supplemental inorganic phosphorus as an ingredient.
Up to 80% of the phosphorus (P) present in plant foods and feeds exists as a complex of phytic acid (myoinositol hexaphosphate), hereinafter referred to as phytate. Phytate may structurally be illustrated by the following formula: ##STR1## The P in phytate cannot be totally digested by simple-stomached animals, including humans, and it therefore passes through the gastrointestinal (GI) tract and is excreted in the feces. In animal nutrition, this is accounted for in diet formulation whereby 1.5 to 2.0% of an inorganic phosphate source is supplemented to meet the animal's minimal P requirement. Addition of inorganic P to poultry, swine, companion animal, and fish diets is expensive. It is often stated that supplemental P for these species is the third most expensive dietary ingredient, after energy and protein. The body requires P for formation of bones and teeth, for phospholipid (cell membrane structure) and nucleic acid (RNA, DNA) synthesis, for synthesis of ATP and other high-energy P compounds, and for proper acid-base balance in the body. Roughly 85% of I.the body P is in the skeleton. Bone is comprised of 50% organic matrix (protein in the form of collagen, and lipid) and 50% inorganic material (mostly a Ca-P salt. i.e., hydroxyapatite).
Supplemental inorganic P is provided to animal diets in one of three feedgrade forms; dicalcium phosphate (18.5% P), monocalcium phosphate (21.5% P) or deflorinated phosphate (18.0% P). The combined total market for these products is estimated to be 675 million dollars per year in the U.S., Canada, Mexico, Western Europe and Japan. If one were to include South America, Eastern Europe, Asia, Africa, China, India, and Southeast Asia, (where market data are difficult to obtain), the total market for feed-grade phosphates could easily be expected to exceed 1 billion dollars annually. In North America, 50% of feed-grade phosphate consumed is used for poultry feeding. It has been discovered that use of a carboxylic acid as an ingredient in animal feed would severely reduce the need for supplemental inorganic P in animal feed, and if combined with a bioactive 1.alpha.-hydroxy vitamin D compound and/or the enzyme phytase, could completely eliminate the need for supplement inorganic P in animal feeds.
Phytate complexes in plant foods and feeds (eg., cereal grains and by-products, beans) also bind cations such as calcium, potassium, magnesium, zinc, iron and manganese (Erdman, 1979) illustrated schematically as follows: ##STR2## A feed additive such as the carboxylic acids and/or bioactive 1.alpha.-OH vitamin D compounds disclosed herein that causes the increased utilization of P from phytate should also increase utilization of these other elements as well. The present invention has established that a carboxylic acid, or a combination of a carboxylic acid with 1-.alpha.-OH vitamin D compounds, preferably 1,25 dihydroxycholecalciferol and 1-.alpha.-OH cholecalciferol, and/or phytase increases the utilization of not only P but also zinc, iron and manganese. Thus, because these three trace elements are always added in supplemental form to diets for swine, poultry and companion animals (as feed-grade ZnO or ZnSO.sub.4 .cndot.H.sub.2 O; FeSO.sub.4 .cndot.H.sub.2 O; MnO or MnSO.sub.4 .cndot.H.sub.2 O) use of a carboxylic acid, or a combination of a carboxylic acid with a bioactive 1-.alpha.-OH vitamin D compound and/or phytase would lower, or perhaps eliminate, the need for supplemental quantities of these mineral salts in a practical-type grain-oilseed meal diet.
By reducing or eliminating the inorganic P supplement and the supplement of trace mineral salts, the remaining diet would contain more usable energy. Thus, grain-oilseed meal diets generally contain about 3,200 kcal metabolizable energy per kilogram of diet, and mineral salts supply no metabolizable energy. Removal of the unneeded minerals and substitution with grain would therefore increase the usable energy in the diet.
Currently, phytase is being used in much of Europe and Asia to reduce P pollution. The use level, however, is 600 units per kilogram diet, but this level was selected because of cost of the enzyme and not because 600 units will maximize phytate utilization. In contrast it has been discovered via the present investigation that at least 1200 units/kg diet is required to maximize phytate utilization in chicks fed a corn-soybean meal diet (Table 1). However, use of a bioactive 1-.alpha.-OH vitamin D compound in accordance with the present invention would reduce the need to feed expensive levels of phytase. (Table 5)
Animal producers are forced to feed high P diets because of the phytate content of diets. This increases P in the excreta waste products (both feces and urine). Excess P from animal, as well as human waste, is generally spread on the soil, where a portion of it gets washed into ground water and then into ponds, streams, rivers, lakes and oceans. Too much P in water stimulates growth of algae, and algae take up considerable oxygen. This robs marine life of the oxygen they need to grow, reproduce and thrive.
In many parts of Europe and Asia, P pollution has become such a problem and concern that penalties in the form of stiff financial fines are imposed on livestock producers who spread too much P-laden manure on the soils. Because of this, much of Europe now uses a microbial phytase product (BASF), even though this product (which also hydrolyses phytate) is very expensive, in fact too expensive to be cost effective (at 600 units/kg diet) as a feed additive in the U.S. at the present time. Many U.S. soils are being described as "P saturated", thus resulting in a greater concentration of P in soil leachates. High-P water leachate in areas such as the Chesapeak Bay has been blamed for excessive algae growth and increased fish kills in bay waters (Ward, 1993). In Europe, the feed industry group FEFANA issued a position paper in 1991 entitled "Improvement of the Environment". They proposed that P in manure from livestock production should be reduced by 30% (Ward, 1993). The limits of P that can be applied to soils in Europe have been discussed by Schwarz (1994). Accordingly, it is estimated that use of a carboxylic acid, or a combination of a carboxylic acid with a 1-.alpha.-OH vitamin D compound that is active in increasing phosphorus utilization in accordance with the present invention, could cut the P content of animal waste products by up to 80%.
Initial work focused on use of 1,25 dihydroxycholecalciferol (1,25-(OH).sub.2 D.sub.3) in the absence or presence of 1200 units of microbial phytase (BASF), Edwards (1993) showed that 1,25-(OH).sub.2 D.sub.3 is effective in improving P utilization from phytate-bound P, and Biehl et al (1995) confirmed his results. Moreover, both studies showed that 1,25-(OH).sub.2 D.sub.3 works additively with microbial phytase in releasing P from dietary phytate complexes. It seems likely that 1,25-(OH).sub.2 D.sub.3 exerts is effects in two ways: (a) the 1,25 compound likely increases the activity of intestinal phytases or phosphatases that hydrolyze phytate (Pileggi et al, 1955; Maddaiah et al, 1964) and (b) the 1,25 compound is known to stimulate phosphate transport (Tanka and DeLuca, 1974), facilitating transport of P from GI tract to plasma and hence bone.
Under normal dietary circumstances, cholecalciferol (vitamin D.sub.3) that is added to a diet gets absorbed from the GI tract and is transported via blood to the liver where the liver enzyme 25-hydroxylase acts on the compound to cause formation of 25-OH D.sub.3. This compound is the normal blood metabolite of cholecalciferol. A small portion of 25-OH D.sub.3 undergoes a further hydroxylation step in the kidney, at the 1-.alpha. position, causing synthesis of the calciotropic hormone 1,25-(OH).sub.2 D.sub.3. Because 1,25-(OH).sub.2 D.sub.3 is expensive to synthesize and because oral 25-OH D.sub.3 is not the active form in phosphate absorption, it was proposed that 1-.alpha.-OH D.sub.3 would be an effective compound for increasing phosphate utilization. It has been discovered that 1.alpha.-hydroxylated vitamin D compounds and particularly 1-.alpha.-OH D.sub.3 will be absorbed from the GI tract and then be transported to the liver where 25-hydroxylase would act upon it to bring about synthesis of 1,25-dihydroxylated compounds and particularly 1,25-(OH).sub.2 D.sub.3. A portion of these compounds would then be transported back to the GI tract where they would activate intestinal phosphate absorption. The net effect would be an increased utilization of P (also Zn, Fe, Mn and Ca) from the phytate complex as well as from the inorganic P supplement itself.
In summary, the potential benefits of the present invention include (1) reduction in or possible elimination of the need for inorganic P supplements for animal (including fish) diets; (2) reduction in P pollution of the environment; (3) reduction or possible elimination of the need for supplemental Zn, Mn and Fe in animal diets; and (4) reduction of the quantity of phytase needed for maximal P utilization from feeds.